Energy-constrained wireless communication systems, such as, for example, battery operated wireless sensor networks, may conserve energy by attempting to keep whenever possible the transceiver in the lowest power mode (e.g., sleep mode). However, in certain wireless communication systems that require low latency for high priority data transfer, it may be important that the links and devices be supervised to better ensure they are operational and usable. This may not only be a regulatory requirement in some systems but also a requirement of the communication protocol.
Wireless sensor networks may be classified into three types of systems based on the type of packet exchange: periodic, reactive or hybrid. Many or most sensor systems are hybrid systems requiring very low latency for high priority data (e.g., alarms in the system), periodic supervision of the error-free operation of the sensors, and a reliable communication link between the sensor and the user. The hybrid systems may include, for example, systems that conform to the Adaptive sensitive Threshold Energy Efficient sensor Network (APTEEN) protocol.
Certain wireless communication systems may supervise links and/or devices by uni-casting complete packets (e.g., a poll) and waiting for an acknowledgement. In this regard, the receiver of the unicasted packets may synchronize to the transmitter and send an acknowledgement that contains the status of the device, as discussed, for example, in German Published Patent No. 199 41 580. If, for example, the transmitter does not receive the acknowledgement, the transmitter may retransmit the packet a pre-defined number of times. If, however, the transmitter receives the acknowledgement, it does not poll the receiver again. To receive the poll packets, the receiver may wake up in every time slot (assigned to it) to check for the poll until a timeout occurs, after which the receiver goes back to sleep. The receiver may also wake up in the next time slot assigned to it and check for a poll, and if it does not receive a poll it may assume the transmitter received the acknowledgement and returns to sleep. This approach, however, may not be suitable for a lossy channel, and may be inefficient in terms of energy and time since complete packets are exchanged without extra information.